Definition: You are given a question on virology. You are also given 4 answer options (associated with "A", "B", "C", "D"), out of which only one is correct. You need to answer the question by selecting the correct option. You should only answer with the choice letter, not the whole answer.
Input: What has so far prevented polio being eradicated from the world?
(A)Inability to vaccinate in some countries (B)Poor quality (unsafe) vaccines (C)Genetic variation (D)Expense

A